Spiral is one of the three Survival gamemode maps in which one to four players try to survive nine waves of neutral units trying to kill their fortress. There are two lanes that the units will move down, as well as a handful of aerial units that fly directly to your fort.
An extremely effective strategy is to put tanks in front of turrets, with artillery units behind them, and a field of Stasis Mines in front. The turrets stop the Tanks from pushing past the defenses, and the Stasis Mines give the artillery more time to shoot the incoming waves of units. This has to be done evenly on both sides, or the units will break through on the weak side.